just how important is medium/water pH?

greentoe357September 3, 2013

When I get a new plant, I google its care and make notes for light, temp, humidity/watering needs, fertilizer advice, how to propagate, when to repot etc. Research is always a fun part of caring for house plants, I think. However, I almost never come across mentions of a plant's preferences for alkaline/acidic environments - generally only if I stumble upon commercial propagation/growing documents or for a very short list of plants where the effect is obvious (blueberries like low pH, hydrangeas turn pink or blue depending on the acidity - that sort of thing). Should a hobby home grower like me with 30-35 various plants even be looking harder? Just how important is pH of water and the medium for your typical home plant?

I grow mostly tropical foliage, all indoors. I have been adding vinegar to my water just because *generally* I understand plants like more acidic water, while municipalities including mine keep their water more alkaline to increase the life of supply pipes and other equipment (to reduce corrosion basically).

I was just thinking of getting pH testing strips or a solution and to see how much vinegar would bring my water pH down to the ideal low-5-ish, but then I thought, while an interesting chemistry experiment, how much am I really fooling myself with how important this stuff is objectively.

wouldn't it be wiser if you just get some bags of lab tested soil from the start and freely remove as much as worn-out soil as possible when re-potting plants to refresh soil medium for good if you have a small collection of potted plants?
you could also flush the plants with water as regular practice whenever possible to prevent possible build-up of salt and leech excesses.

the ideal soil pH for optimal growth in most plants is 6.5-7. most plants can adapt in wide range of pH values but there are also those temperamental plants.

I believe the question is about water PH, not the soil? If so, I can tell you that switching to rain water has made an extremely noticeable difference in a lot of plants. However, it's impossible to isolate PH from the chemicals added to tap water, like fluoride and the chlor's, so also impossible to say which factor is emphasized, or if the combination of removing both hazards is what is so effective. So I avoid tap when there is rain water to catch, except while plants are inside for winter, then that's what they get.

A dehumidifier, if you have one, is a convenient source of distilled water.

If you water plants in sips, the mineral content of the water (whatever it might be, including fertilizer) will accumulate in the soil -- affecting pH as well as the mineral concentrations themselves. If you water enough so that some flows out the bottom, it won't.

If you fertilize when you water, I would guess that the buffering capacity of the phosphate in the fertilizer will probably outweigh that of your tap water.

I do not think so for many reasons. Mostly because most commercial mixes are too water-retentive for me (but let's not get into the mix composition discussions in this pH thread). Another major reason is the fact that manufacturers test their mixes for pH does not mean it's going to remain fixed - watering, fertilizing, soil decomposition all change the mix pH over time.

I do get rid of old soil when repotting though, as you suggest. And I do leech my mix regularly (let the water/fertilizer solution drain freely at every watering).

>> ideal soil pH for optimal growth in most plants is 6.5-7

Hmm. I've read 5-5.5 somewhere. Need to look into it more and appreciate references.

>> I believe the question is about water PH, not the soil?

Both. Water/fertilizer affect pH of the mix over time. Although roots absorb the solution, not the mix, so the water pH is more important, I guess.

Rain water is not available to me, as I live in an apartment with no balcony. I do let tap water stand in a bucket for several days before using. The temp equalizes and most of chlorine evaporates, although I understand fluoride doesn't.

Not sure how workable the dehumidifier idea is. Definitely not outside of summer, as I use a humidifier then.

>> If you fertilize when you water, I would guess that the buffering capacity of the phosphate in the fertilizer will probably outweigh that of your tap water.

I do not know what this means, sorry.

Thanks all, but I think the question still remains: how important is trying to control the mix and water/fert solution pH?

When I was a student there were these factors like cation exchange capacity or buffer capacity that made the pH of the water nearly irrelevant unless you have some really bad water OR you are into hydroponics OR you have commercial or contest aspirations.

I feel so bad about making such a strong statement above. People grow fine plants with tap water all of the time, I certainly did until recently, so it's certainly rarely, if ever, a matter of life and death. If there's not much you can or want (spending money) to do about it, why worry anyway? It's your time, money, so spend it however you want, but that's my take on it, just chill out and see what does well here. Without reliable equipment to know what you're doing, seems impossible to do anyway, just so much guess work based on extrapolated assumptions. Are you seeing chlorotic, yellowed leaves? That would be a reason for such concern or investigation, otherwise I would think, if it ain't broke, don't fix it. Fertilizer is usually the answer, as said, if there is such a problem that a plant is ill, not just a little slow.

Some plants just aren't going to grow well in some environments, so knowing when to try something different is also good. Temp, humidity, tap water for winter - things plants just have to deal with or they die here. I could fill 10 houses with all different plants that would like it just fine, so the others, oh well.

The standard of chlorinating water used to use a volatile form, but most municipalities have switched to a form that does not evaporate so that advice is kind of old-school, although I do let tap water sit when using it over winter so it's not so cold, like you said.

Generally, plant advice tends to be full of arcane references and "no way I'm doing that" instructions that sound overly difficult or complicated, or uses stuff you can't get. If you don't want to, or just can't do something, it's probably not necessary. The key to having a bunch of plants you like that are doing well is often knowing which advice to ignore. For example, I've never used a humidity dome or tent, but have propagated things thousands of times. I've never double-dug a new bed, or amended a hole for a tree or shrub. But yes, succulents really do better if they callous over first... Plants are very forgiving, for every fail, there are 17 other ways it could have went OK.

Pic of tree below (Dracaena marginata, known to be sensitive to about anything in tap water,) is from 10 years ago, at which point it was already decades old, having been guzzling plenty of tap water, at least during winters, and whenever mother nature didn't provide enough rain during summer. It's still alive and well today, both the original trunk/roots, and many tops separated and propagated into their own little trees.

Soil structure, IMHO/E, closely followed by having enough light, is much more important than whether or not the water is tap, and much easier to control, for those with cold seasons or no access to rain water. If you're just curious, that's cool, but getting frustrated about stuff that's out of your control kind of takes the fun out of things, so I just hope you're not doing that, that's all I'm sayin'. ...

If you have a humidifier and a dehumidifier, there's nothing wrong with running them both at the same time, if you want distilled water in the winter. Just don't empty the dehumidifier except for the water you need, and (if the humidifier is on a humidistat) you'll automatically be running the humidifier enough extra to put the necessary water into the air, and running the dehumidifier only enough to take the same amount of water out of the air. Or run the dehumidifier when you cook a batch of pasta. Or don't worry about it, and just use tap water.

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Buffering capacity is how much acid or base you have to add in order to get the pH to change. If you put a few drops of acid into a gallon of pure water, the pH will go almost all the way down to the pH of the acid. Likewise if you had put in a few drops of a base instead, the pH will go almost all the way up to that of the base. But if you've got a lot of phosphate in the water, the pH may barely change at all.

Different chemicals buffer at different pH. If you've got water with a lot of potassium phosphate and the pH was at 11, it only takes a tiny amount of acid to lower it to 9: phosphate doesn't buffer in that range. But to get it to go past neutral, you need to add an amount of acid comparable to how much potassium phosphate is in the water: phosphate does buffer around neutral pH. It goes both ways: if a phosphate solution is acidic and you start adding a base, it won't get alkaline until you've added about as much of the base as the amount of phosphate.

So your tap water may be alkaline, but they only had to add a tiny bit of base to make it that way. Meanwhile, your fertilizer is probably mildly acidic, and it buffers in the near-neutral pH range, so it will stay mildly acidic when combined with whatever's in your tap water.

If you just add a little fertilizer to the water every time you water, and water enough so that some drains, you shouldn't have to worry about pH except for selecting a fertilizer with suitable pH for your plants.

Theoretically, anyway. I don't know whether it really works out that way in practice.

If you feel the need to lower the pH, then do so..It is very 'important' that the pH of your watering solution makes the nutrients available to your plants at all times.

You will notice a huge difference in vigor and color. If you don't then the vinegar would not of harmed a thing, as much as a tablespoon or so which makes a huge difference if needed, then the problem with your plants is something else to investigate.

If your water pH is 8.0 or higher, then I can almost promise you that your pH has to be adjusted just as I do for all my plants.
The fact that you are even concerned puts you on the right road to beautiful healthy plants. Good job.

Albert, I had bravely but casually approached the "cation exchange capacity" entry on wikipedia once before and soon retreated with my eyes glazed over. I'll have to read around again to see how or why it makes the pH of water nearly irrelevant as you say. Last time I took chemistry was... khm'teen years ago, but I'll do my best. :-)

You mentioned hydroponics as an exception. The medium I grow in ("gritty mix" by Al from "container gardening" forum) is so fast-draining that people say it's very close to hydroponics actually.

Purple, I re-read what you wrote, and it does not look like "too strong" a statement at all. Rain water is indeed better than tap water - and the fact that I do not have a way of collecting it does not change that fact, I am still glad you mentioned it. No worries at all. Thank you actually.

I do not have any chlorotic leaves or fertilizer burn on my plants, but every time I add that tablespoon of vinegar to a gallon of water, part of me thinks if I am really doing something completely unnecessary - or if I should be adding even more. Hence this thread. It's no biggie, no stress here, just a healthy dose of curiosity. :-)

dsws, running a humidifier and a dehumidifier at the same time seems a little extreme to me, and I doubt how effective it's going to be. Winter humidity indoors gets down to 20-30% or even into teens here, and the humidifier blasting full-speed does not really increase it well enough into some of my plants' comfort zone). Then there are energy inefficiency issues, the need to buy another device (more clutter to stab my toes against, yay!) - after already having bought the humidifier last winter exclusively for the plants. It's unlikely I'll go for that - but I do appreciate the idea none the less.

Thanks even more for the excellent explanation of the buffering capacity. Very interesting!

I am glad to hear pH probably does not matter an awful lot in practice, but I am curious... If I understand correctly, what I need to do if I want to know how much vinegar to add to my water is: to let tap water stand around a few days like I normally do, add my usual dose of fertilizer, then measure pH, then if it's too high, add vinegar till pH gets to optimal range , which is either 5-5.5 or 6.5-7 depending who you ask (which is it?) That will give me the amount of vinegar I should be adding, assuming the water composition does not change, which of course it does all the time, but let's ignore that. Does this experiment make sense?

I've been adding a tablespoon of vinegar per gallon, but I've seen dosage recommendations anywhere from a teaspoon to half a cup per gallon. They can't all be right. I do not have enough experience to recognize pH problems if/when they happen, so would like to get this right.

You can head to your local pet shop and buy a PH test kit fairly cheap. draw a gallon (4 liters) of water, test it, add a teaspoon (5 milliliters) of vinegar, stir, let it sit for a minute and test again. Now what's your PH? is it what you want it to be? if not, add another teaspoon. Adjust as needed to get your preferred PH.

Then take your gallon of modified and tested water and let it sit for the usual time you let your water sit before you water your plants and test it again so you can see what sort of buffering capacity your water has. Depending on the minerals that are dissolved in the water it may bounce back up.

If it does you may want to consider using a product that will also buffer the water to the lower PH. You can probably this pick up at a local Hydroponics store, there are several in Brooklyn. They should also be familiar with water issues that are of local concern.

A costlier solution would be to get a small RO (Reverse Osmosis) filtration unit. That would give you more or less neutral water all the time with almost no mineral content that would be easy to adjust to anything you want.

The whole suggestion of using a dehumidifier was premised on "if you have one". I bought one because the humidity would get so high that the towels wouldn't dry, everything was musty, and the door to my apartment would warp in wet weather and be hard to close. Now that I have it, it's a convenient source of distilled water. But I wouldn't have gotten it just for my plants.

"what I need to do if I want to know how much vinegar to add to my water is ..."

For the amount of acid, the thing to look at on the report is not pH, but rather alkalinity. That's the number that tells how much buffering capacity the water has. It's in milligrams of CaCO3 per liter of water, and you want tablespoons of vinegar per gallon of water. CaCO3 and vinegar neutralize each other one-for-one in the pH range we're looking at (I think). But that's one molecule for one molecule, so there's another conversion factor to multiply.

CaCO3 weighs 40+12+3*16 = 100, and CH3COOH weighs 12+3*1+12+16+16+1 = 60; vinegar is 5% CH3COOH; a teaspoon is about 5mL; and dilute solutions weigh about a gram per mL. The alkalinity of your tap water is listed at your link as being 11.2 - 19.3 mg CaCO3 equivalent per liter of water.

greentoe357, if I may add another aspect; the acetic acid in the soil solution will enable the roots of the plant to extract more of the nutrients from the soil.
Long ago and far away, we used to do two soil extracts - water and acetic acid separately. I recall significant differences - especially with respect to iron.